A Treatise On The Principles And Practice Of Harbour Engineering

208 HARBOUR ENGINEERING. subsidiary burners likewise draw their alimentation by means of a bye-pass from tire mixing chamber. The difficulty attaching to the use of incandescent burners is the fragility of the mantles. The average life of an ordinary mantle is perhaps six or seven days. Yet mantles have been known to last for over thirty days with care. The vaporising coils and tubes last from four to six months, and require frequent and constant cleaning. Acetylene has also been used as an illuminant, but only to a very limited extent, although the dangers formerly attending its production have now been largely overcome by the improved nature of the generating cylinders. It is still undesirable to use this illuminant in the highly com- pressed liquid condition, from which explosions have resulted. Dissolved acetylene, however, is largely used in motor cars, and is now being tested by Trinity House. Acetylene in this form certainly gives indications of being the coming illuminant for buoys, etc. Light Concentration.—The source of light being but one of the factors in the determination of lighthouse efficiency, we now turn our attention to the methods adopted for the concentration and intensification of the issuing rays. The simple, undirected flame is wasteful of light; that is, much of the light is lost to useful purposes. Most lighthouses stand upon the coast-line, and the area of radiation, therefore, frequeutly includes a large sector of land over which illumination is entirely unnecessary. Also, apart from this cause, a good deal of light is lost by diffusion and dispersion. To remedy these defects, reflectors were introduced as far back as the latter half of the eighteenth century. At first spherical in form, the mirror ultimately became parabolic, concentrating the emergent rays of the light from the focus along a path parallel to the horizontal axis. This constitutes the catoptric principle. Catoptric reflectors are of two types: first, the paraboloid, formed by the generation of a parabola about its own axis, and send- ing the light rays in a single direction only ; and secondly, the dual (upper and lower) surfaces formed by the horizontal rotation of a parabola round a vertical axis through the focus. This system, while confining the light within vertical limits, distributes it equally throughout a horizontal plane. The dioptrie, or lentieular, principle of ray concentration, based on the refractive properties of lenses, is due to Augustin Fresnel, who initiated it, or rather, applied it in an elementary form in 1822. As then exemplified, it consisted of a plano-convex lens set vertically in front of the light, so that all rays passing through the lens were transmitted horizontally. To the central lens were then added a number of parallel lenses of triangular form, which served to refract a certain proportion of the rays passing above and below it. The amount of non-utilised light was still considérable. Fresnel disposed his lenses so as to form a cylinder completely inclosing the light, which thus illuminated the entire circumference uniformly. Stevenson devised a variation known as the holophotal system, by which the light was surrounded by a series of panels, each containing a circular